The present invention relates to a capacitive sensing array device having a structure comprising an array of sense electrodes carried on a substrate and covered by a layer of dielectric material defining a sensing surface.
A device of the above described kind and intended for sensing capacitively fingerprint patterns in particular is described in U.S. Pat. No. 5,325,442. The device compares a plurality of sense elements arranged in a row and column matrix array. Each sense element consists of a sense electrode connected to an associated switching device in the form of, for example a thin film transistor, (TFT), and the switching devices of the array are connected to a peripheral drive circuit via sets of row and column address conductors carried on the substrate and extending between the sense electrodes. The drain electrode of each TFT is connected to the sense electrode of the sense element. When a finger is placed over the dielectric material, the sense electrodes together with the overlying layer of dielectric material and individual fingerprint portions constitute capacitors. The row address conductors are connected to a scan circuit which applies a selection signal to each row conductor in sequence to turn on the TFTs of the sense elements of the row. Simultaneously with a selection signal a potential is applied to the column address conductors to charge the capacitors. The individual capacitances of these capacitors depend on the spacing of the fingerprint portions from the sense electrodes, as determined by the presence of a ridge of a trough of the fingerprint, and are measured by sensing the charging current flowing in the column conductors during charging of the capacitors, using current or charge sensing amplifier circuits incorporated in the drive circuit. At the end of a row address period, the TFTs are turned off and a gating signal applied to the next row conductor to turn on the TFTs of the next row of sense elements. Each row of sense elements is addressed in this manner in turn and the variation in sensed capacitances produced over the array of sense elements by a fingerprint ridge pattern provides an electronic image or representation of the fingerprint pattern. In addition to the sensing element array being formed using thin film technology, with the transistors comprising thin film transistors (TFTs) on an insulating substrate for example of glass or plastics, the array may alternatively comprise an integrated circuit using a silicon substrate.
In a modification of one embodiment of fingerprint sensing device described in this specification, the structure includes metal grounding conductors provided on the surface of the dielectric layer and overlying the spaces between the sense electrodes, either in a grid pattern or as linear conductors, for the purpose of improving electrical contact to the finger surface.
In a separate embodiment also described, each sense element is provided with a second, electrically isolated, electrode on the surface of the dielectric layer which is of a similar size to the sensing electrode and arranged overlying the sense electrode. The second electrodes are intended in use to be contacted and grounded by ridges of a person""s fingerprint placed thereon so as to define together with their underlying sense electrodes and intervening dielectric material substantially identical, and more distinctive, capacitors at the fingerprint ridge locations.
A similar kind of device to that discussed above, comprising an active matrix array with capacitive sensing electrodes but occupying a larger are and in which the pitch of the sense electrodes is increased could be used as a touch input device such as a graphics tablet, responsive to a person""s finger or a stylus.
A problem with such capacitive sensing array devices is that an electrostatic charge from the body of a person touching the device or even bringing a finger into proximity to the sensing surface can cause damage, particularly to the switching devices which are susceptible to such charges.
It is an object of the present invention to provide an improved capacitive sensing array device which is less susceptible to damage being caused in this manner.
According to the present invention, there is provided a capacitive sensing array device of the kind described in the opening paragraph wherein grounding conductors are provided adjacent to the sensing surface and extending alongside at least one edge of each of the sense electrodes, and wherein the material of the dielectric layer at least at the region of the grounding conductors comprises a semi-insulating material having a non-linear current-voltage characteristic whereby electrostatic charges transferred to, or induced in, the device structure in use are conducted through the semi-insulating material to the grounding conductors.
The risk of damage being caused to the device, and especially components such as the switching devices (e.g. transistors) associated with the sense electrodes, through electrostatic charges carried by a person when touching, or bringing a body part into proximity with, the device is considerably reduced. The invention involves recognition that while the conductors on the sensing surface as described in U.S. Pat. No. 5,325,442 can serve to dissipate electrostatic charges carried on a person""s finger when the finger touches the conductors, the behaviour, and particularly the position, of electrostatic discharges especially when a finger is being brought close to the device is highly unpredictable and because the area of the conductors is relatively small compared with the area occupied by the sense electrodes such charges can, for example, easily by-pass the conductors and pass through the dielectric layer to a sense electrode and from there to the switching devices. By using a highly non-linear material adjacent the grounding conductors, electrostatic charge effectively is encouraged to pass to a grounding conductor instead due to the behaviour of this material at higher voltages. As the potential between the grounding conductor and the region where the discharge occurs rises the impedance of this material therebetween falls, thereby allowing the charge to be conducted away in a controlled manner.
The highly non-linear semi-insulating material preferably comprises a non-stoichiometric (silicon-rich) amorphous silicon alloy, such as silicon nitride, silicon carbide, silicon oxide, or silicon oxynitride, or alternatively tantalum oxide, the behaviour of such materials in this respect being well known. Such materials have been employed in two terminal non-linear device applications such as thin film diodes, (TFDs), which term is used herein to include MIM (metal-insulator-metal) devices, used as bi-directional, two-terminal switching elements in active matrix display devices and the like. These materials are normally insulating but their conductivity is a strong function of applied electric field. They exhibit a threshold characteristic and start to conduct once the voltage thereacross reaches the threshold level (of either polarity). Of these materials, non-stoichiometric, (silicon-rich) hydrogenated amorphous silicon nitride is particularly preferred in view of the highly controllable, and predictable, non-linear characteristics of this material. Known non-linear materials other than those specifically mentioned and used for similar purposes and including certain polymers could also be suitable.
In one preferred embodiment, the grounding conductors are provided below the sensing surface and each sense electrode is associated with second electrode which is carried on the sensing surface overlying the sense electrode and whose edge is in close proximity to a grounding conductor, and at least the region of the dielectric layer between the grounding conductors and the sensing surface comprises said semi-insulating material. With this structure, any voltage excursion on the second electrode beyond a predetermined threshold level and resulting from a charged finger or the like touching or approaching that electrode will cause the impedance of the non-linear material between the electrode and a grounding conductor to fall allowing the charge to be conducted therethrough to the grounding conductor. Preferably, the edge of the second electrode is arranged to overlap a grounding conductor. At the overlap region a conductor/non-linear-dielectric/conductor structure similar to that of known thin film diodes is obtained with the thickness of the non-linear material between the two conductors corresponding to the thickness of the upper portion (sub-layer) of the dielectric material. As this thickness determines the threshold level and can be well-defined in the deposition processes used, the operational characteristics of the thin film diode structure formed thereby, and especially its threshold level, can easily be made to be highly controllable and predictable.
In another preferred embodiment the grounding conductors are carried on the sensing surface and the material of the dielectric layer between the sense electrode and the sensing surface comprises said semi-insulating material. In this structure, the dielectric layer becomes conducting once the voltage across it rises above a critical, threshold, value, and so when the voltage on the sense electrode increases to an excessive level, as a result of the effects of electrostatic charges on a person""s finger, the charge is conducted away through this layer to the grounding conductor. The increase in the voltage on the sense electrode can be caused by capacitive coupling effects upon an electrostatic charge being present at or near the sensing surface or by actual breakdown of the dielectric material allowing such charge to pass to the electrode. The level at which this charge is conducted to a grounding conductor is selected to be below a level at which damage can be caused to a switching device coupled to the sense electrode.
In both preferred embodiments, therefore, voltage excursions occurring on the sense electrodes are limited, and consequently the risk of damage being caused to components connected electrically thereto is minimised. At lower voltages, impedance to ground via the semi-insulating material is comparatively very high so normal operation of the device is unaffected.
The grounding conductors may extend linearly between adjacent rows or columns of the sense electrodes. Preferably though, the grounding conductors are provided in the form of a grid with each sense electrode being surrounded by a grid portion.
Although the invention is particularly beneficial in capacitive type fingerprint sensing devices of the kind for example described in U.S. Pat. No. 5,325,442, it of course can be beneficially applied in other kinds of capacitive sensing array devices where similar problems with electrostatic charge may be experienced.